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Techniques", run by the Health Section of the
International Atomic Energy Agency (IAEA). The
participants of the audit were obliged to irradiate
provided dosimetric films, in a slab phantom, for a specific
leaf arrangement, producing a pattern of five stripes,
commonly called a picket fence. The participants had to
programme such a pattern so that the stripes are 5 mm
wide and are 3 cm distant between themselves. The
Gafchromic EBT2 radiochromic films were placed in a slab
phantom close to maximum dose depth. The irradiation
was 250 MU per stripe.
Results
Thirty two Polish radiotherapy centres took part in the
audit. They were equipped with various accelerator types
and various treatment planning systems. In all cases the 6
MV quality beams were used. The discrepancies between
measured and expected stripe positions were in the range
1.2 mm. For particular participants, the leaf position
discrepancies were in the range -0,5 mm to 0,5 mm. For
particular participants, the mean opening width measured
with films for each pair of leafs was between 6 and 8 mm.
Conclusion
In the audit, the best performance showed the new type
multileaf collimators with 120-160 leafs, The worst
performance showed collimators MLC80 from Elekta. The
results of the audit are very useful for the participants
who should carefully investigate the performance of their
multileaf collimators.
EP-1741 Commissioning of a robotic patient positioning
system equipped with an integrated tracking system
A. Ableitinger
1
, A. Utz
1
, A. Zechner
1
, S. Vatnitsky
1
, M.
Stock
1
1
EBG MedAustron GmbH, Strahlentherapie, Wiener
Neustadt, Austria
Purpose or Objective
Robotic patient positioning systems (PPS) used in clinic
must consider weight-induced couch bending and must
show high reproducibility and stability to achieve the
required positioning accuracy. Extensive commissioning of
these robotic systems is therefore crucial. The aim of the
current work is to determine the positioning accuracy of
the PPS, that is equipped with an integrated optical
tracking system.
Material and Methods
Three different aspects of the PPS were investigated in
this study: the basic characteristics including couch
bending, reproducibility and stability; the relative spatial
deviation in terms of rotation and couch height and the
absolute accuracy of the treatment couch.
The treatment volume of the PPS has a dimension of
115cm x 50cm x 40cm. The robotic system enables couch
rotations of more than 190°, pitch and roll of ±3° and non-
isocentric treatment positions. A photogrammetric
camera tracks the treatment couch of the PPS via
reflecting markers mounted on the bottom side of the
couch (see Fig.1). An iterative position correction loop
aligns the couch to the prescribed position.
The reference instrument was a laser tracker with
reflecting probes. Drilling holes near the indexing
positions (H4-F9) located laterally along the couch every
14cm served as measurement positions for the evaluation
of the basic characteristics and the absolute couch
position. For the relative deviation the drift of one
measurement point on the couch was evaluated.
To determine the bending of the couch it was loaded with
six different weights up to 156kg. The reproducible
positioning for the same couch position (different axis
setting) and the couch stability after 1 hour were
measured with the highest payload.
The evaluation of the absolute spatial deviation was based
on six measurement points being closest to the room
isocenter. They were compared with their expected
coordinates for 1020 different robot positions, poses and
payloads.
Results
The
basic
characteristics
of
couch-bending,
reproducibility and stability were within 0.10mm within
the treatment volume (see Fig.2 a, b).
The relative spatial deviation was smaller than 0.40mm
for rotations ranging up to 200° (see Fig.2 c). For different
vertical positions, the couch drifted less than 0.25mm for
2 different loads and rotations.
The differences between the prescribed and measured
absolute position were evaluated in terms of histograms
showing the overall 3D deviation. In 95% of all
measurement points the 3D accuracy was better than 0.63
mm (see Fig.2 d).
Regarding the weight-induced couch-bending no
correlation between the accuracy and payload could be
found.
Conclusion
The results show that the important spatial properties of
the patient positioning system are well within the
acceptable clinical tolerances. The very high
reproducibility of the PPS allows further optimization of
the absolute position. The measured datasets serve as
new input for a high accuracy calibration.
EP-1742 Optimisation and implementation of brain
CBCT templates; an institutional pilot study.
S. Petkar
1
, N. Lalli
2
, F. Solda'
1
, C. Gillies
2
, S. Moinuddin
1
,
N. Fersht
1
1
UCLH NHS Foundation Trust, Radiotherapy, London,
United Kingdom
2
UCLH NHS Foundation Trust, Radiotherapy Physics,
London, United Kingdom
Purpose or Objective
Volumetric imaging (CBCT) in brain has facilitated the use
of volumetric delivery techniques and reduction of